Method for preparing melamine

ABSTRACT

Method for preparing melamine from urea via a high-pressure process in which solid melamine is obtained by transferring the melamine melt coming from the reactor to a vessel where the melamine melt is cooled with an evaporating cooling medium. The melamine melt comes from the melamine reactor at a temperature between the melting point of melamine and 450° C. and is sprayed into a cooling vessel, via spraying means, having an ammonia environment with an increased ammonia pressure and cooled by means of an evaporating cooling medium to form melamine powder. The melamine melt is thereby converted into melamine powder having a temperature of 270° C. or below after which the ammonia pressure is released and the melamine powder is cooled, at least for part of the cooling range, by the powder being set in motion mechanically and being cooled directly or indirectly and, if necessary, the melamine powder is cooled further.

This is a continuation of PCT/NL98/00281, filed May 15, 1998. Thisapplication claims the benefit of U.S. Provisional Application No.60/048,478, filed Jun. 3, 1997.

The invention relates to a method for preparing melamine from urea via ahigh-pressure process in which solid melamine is obtained bytransferring the melamine melt coming from the reactor to a vessel wherethe melamine melt is cooled with an evaporating cooling medium.

Such a method is disclosed, inter alia, in EP-A-747366 which describes ahigh-pressure process for preparing melamine from urea. In particular,EP-A-747366 describes how urea is pyrolyzed in a reactor, operating at apressure of from 10.34 to 24.13 MPa and a temperature of from 354 to454° C., to produce a reactor product. This reactor product, containingliquid melamine, CO₂, and NH₃ and is transferred under pressure as amixed stream to a separator.

In this separator, which is kept at virtually the same pressure andtemperature as the reactor, the reactor product is separated into agaseous stream and a liquid stream. The gaseous stream containsprimarily CO₂ and NH₃ waste gases and melamine vapor. The liquid streammainly comprises a melamine melt. The gaseous stream is transferred to ascrubber unit, while the liquid stream is transferred to aproduct-cooling unit.

In the scrubber unit, operated at temperature and pressure conditionsnearly identical to the reactor conditions, the gaseous stream isscrubbed with molten urea. The heat transfer achieved in the scrubberunit both preheats the molten urea and cools the gaseous stream to atemperature from 177 to 232° C. The molten urea also scrubs the gaseousstream to remove the melamine vapor from the waste gases. The preheatedmolten urea, along with the melamine that was scrubbed from the CO₂ andNH₃ waste gases, is then fed into the reactor.

In the product-cooling unit, the melamine melt is cooled and solidifiedwith a liquid cooling medium to produce a solid high purity melamineproduct without the need for additional purification. The preferredliquid cooling medium is one that forms a gas at the temperature of themelamine melt and at the pressure in the product-cooling unit.EP-A-747366 identifies liquid ammonia as the preferred liquid coolingmedium with the pressure in the product-cooling unit being above 41.4bar. Although according to EP-A-747366 the purity of the solid melamineproduct obtained using the disclosed process was greater than 99 wt %,this degree of purity has proven difficult to maintain continuously on acommercial scale. The inability to maintain a purity greater than 99 wt% is a drawback that renders the melamine produced less suitable formore demanding applications, particularly melamine-formaldehyde resinsused in laminates and/or coatings.

The object of the present invention is to obtain an improved method forpreparing melamine from urea, in which melamine is obtained directlyfrom the reactor product as a dry powder having a high degree of purity.More particularly, the object of the present invention is to obtain animproved high-pressure process for preparing melamine from urea, inwhich melamine is obtained directly from the liquid melamine melt as adry powder having a high degree of purity via cooling.

Applicant has found that high purity melamine can be continuouslyproduced from the melamine melt coming from the melamine reactor, whichhas a temperature between the melting point of melamine and 450° C., byspraying the melamine melt via spraying means into a vessel and coolingthe melamine melt by contact with an evaporating cooling medium in anammonia atmosphere with an increased ammonia pressure whereby melaminepowder is obtained with a temperature below 270° C., releasing theammonia pressure and cooling the melamine powder, at least for part ofthe cooling range, by agitating the powder mechanically and coolingeither directly, indirectly or some combination.

An increased ammonia pressure means an ammonia pressure greater than 1MPa, preferably greater than 1.5 MPa, more preferably greater than 4.5MPa and even more preferably greater than 6 MPa. The ammonia pressure isbelow 40 MPa, preferably below 25 MPa and more preferably below 11 MPa.

Melamine powder has poor flow and fluidization characteristics and a lowtemperature equalization coefficient (poor thermal conductivity).Standard cooling methods such as a fluidized bed or a packed moving bedcannot, therefore, be readily implemented on a commercial scale. We havefound, however, that the color of the melamine powder, in particular, isadversely affected if the melamine remains at a high temperature for toolong. Effective control of the residence time at high temperature has,therefore, proved critical. It is therefore important to be able to coolthe melamine powder effectively.

Surprisingly, it proved possible to cool melamine powder, despite itspoor flow and thermal conductivity characteristics, by utilizingmechanical agitation coupled with direct and indirect cooling. The termindirect cooling describes those instances in which the mechanicallyagitated melamine powder contacts a cooled surface. The term directcooling describes those instances in which the mechanically agitatedmelamine powder contacts a cooling medium such as ammonia or anairstream. A combination of both direct and indirect cooling mechanismsis obviously also possible.

The melamine powder formed by spraying the melamine melt into thesolidification vessel is held under an increased ammonia pressure at atemperature above 200° C. for a contact time. The duration of thiscontact time is preferably between 1 minute and 5 hours, more preferablybetween 5 minutes and 2 hours. During this contact time, the temperatureof the melamine product can remain virtually constant or it may becooled to a temperature above 200° C., preferably above 240° C., or,most preferably, above 270° C. An increased ammonia pressure means anammonia pressure greater than 1 MPa, preferably greater than 1.5 MPa,more preferably greater than 4.5 MPa and even more preferably greaterthan 6 MPa. The ammonia pressure is below 40 MPa, preferably below 25MPa and more preferably below 11 MPa. The melamine product may be cooledin the solidification vessel or in a separate cooling vessel.

The advantage of the method according to the present invention is thecontinuous production, on a commercial scale, of dry melamine powderwith a purity above 98.5 wt %, and generally above 99 wt %, that hasvery good color characteristics. The high purity melamine producedaccording to the present invention is suitable for virtually anymelamine application, including melamine-formaldehyde resins used inlaminates and/or coatings.

The preparation of melamine preferably uses urea as the raw material,the urea being fed into the reactor as a melt and reacted at elevatedtemperature and pressure. Urea reacts to form melamine, and theby-products NH₃ and CO₂, according to the following reaction equation:

6CO(NH₂)₂→C₃N₆H₆+6NH₃+3CO₂

The production of melamine from urea can be carried out at highpressure, preferably between 5 and 25 MPa, without the presence of acatalyst, at reaction temperatures between 325 and 450° C., andpreferably between 350 and 425° C. The by-products NH₃ and CO₂ areusually recycled to an adjoining urea factory.

The above-mentioned objective of the invention is achieved by employingan apparatus suitable for the preparation of melamine from urea. Anapparatus suitable for the present invention may comprise a scrubberunit, a reactor having either an integrated gas/liquid separator or aseparate gas/liquid separator, possibly a post-reactor, a first coolingvessel, and possibly additional cooling vessels. When a separategas/liquid separator is used, the pressure and temperature of theseparator are virtually identical to the temperature and pressure in thereactor.

In an embodiment of the invention, melamine is prepared from urea in anapparatus comprising a scrubber unit, a melamine reactor having eitheran integrated gas/liquid separator or a separate gas/liquid separator, afirst cooling vessel, and a second cooling vessel. In this embodiment,the urea melt is fed into a scrubber unit operating at a pressure offrom 5 to 25 MPa, preferably from 8 to 20 MPa, and at a temperatureabove the melting point of urea. This scrubber unit may be provided witha cooling jacket or internal cooling bodies to provide additionaltemperature control.

As it passes through the scrubber unit, the urea melt contacts thereaction waste gases coming from the melamine reactor or the separategas/liquid separator. The reaction gases mainly consist of CO₂ and NH₃and may include melamine vapor. The urea melt scrubs the melamine vaporfrom the CO₂ and NH₃ waste gases and carries this melamine along back tothe reactor. In the scrubbing process, the waste gases are cooled fromthe temperature of the reactor, i.e. from 350 to 425° C., to from 170 to240° C., the urea being heated to from 170 to 240° C. The CO₂ and NH₃waste gases are removed from the top of the scrubber unit and may, forexample, be recycled to an adjoining urea factory, where they can beused as raw materials for the urea production.

The preheated urea melt is drawn off from the scrubber unit, togetherwith the melamine scrubbed from the waste gases, and transferred to thehigh pressure reactor operating at pressures between 5 and 25 MPa, andpreferably between 8 and 20 MPa. This transfer may be achieved using ahigh-pressure pump or, where the scrubber is positioned above thereactor, gravity, or a combination of gravity and pumps.

In the reactor, the urea melt is heated to a temperature between 325 and450° C., preferably between about 350 and 425° C., under a pressurebetween 5 and 25 MPa, preferably between 8 and 20 MPa, to convert theurea into melamine, CO₂, and NH₃. In addition to the urea melt, acertain amount of ammonia can be metered into the reactor as, forexample, a liquid or hot vapor. The additional ammonia, althoughoptional, may serve, for example, to prevent the formation ofcondensation products of melamine such as melam, melem, and melon, or topromote mixing in the reactor. The amount of additional ammonia suppliedto the reactor may be up to 10 moles ammonia per mole of urea,preferably up to 5 moles ammonia per mole of urea, and, most preferably,up to 2 moles of ammonia per mole of urea.

The CO₂ and NH₃ produced in the reaction, as well as any additionalammonia supplied, collect in the separation section, for example in thetop of the reactor or in a separate gas/liquid separator positioneddownstream of the reactor, and are separated from the liquid melamine.If a separate, downstream gas/liquid separator is used, it may beadvantageous for additional ammonia to be metered into this separator.The amount of ammonia in this case is 0.01-10 moles of ammonia per moleof melamine, and preferably 0.1-5 moles of ammonia per mole of melamine.Adding additional ammonia to the separator promotes the rapid separationof carbon dioxide from the reactor product, thus preventing theformation of oxygen-containing by-products. As described above, the gasmixture removed from the gas/liquid separator may be passed to thescrubber unit in order to remove melamine vapor and preheat the ureamelt.

The melamine melt, having a temperature between the melting point ofmelamine and 450° C., is drawn off from the reactor or from thedownstream gas/liquid separator and sprayed into a cooling vessel toobtain the solid melamine product. Prior to spraying, however, themelamine melt may be cooled from the reactor temperature to atemperature closer to, but still above, the melting point of melamine.

The melamine melt will be drawn off from the reactor at a temperaturepreferably above 390° C., and more preferably above 400° C., and will becooled at least 5° C., and preferably at least 15° C., before sprayinginto the cooling vessel. Most preferably the melamine melt will becooled to a temperature that is 5-20° C. above the solidification pointof melamine. The melamine melt may be cooled in the gas/liquid separatoror in a separate apparatus downstream of the gas/liquid separator.Cooling can take place by injection of a cooling medium, for exampleammonia gas having a temperature below the temperature of the melaminemelt, or by passing the melamine melt through a heat exchanger.

Furthermore, ammonia can be introduced into the melamine melt in such away that a gas/liquid mixture is sprayed in the spraying means. In thiscase, the ammonia is introduced at a pressure above that of the melaminemelt and preferably at a pressure between 15 and 45 MPa.

The residence time of the melamine melt between the reactor and thespraying means is preferably at least 10 minutes, and most preferably atleast 30 minutes, and usually less than 4 hours.

The melamine melt, possibly together with ammonia gas, is transferred toa spraying means where it is sprayed into a first cooling vessel tosolidify the melamine melt and form a dry melamine powder. The sprayingmeans is an apparatus by which the melamine melt stream is convertedinto droplets, by causing the melt to flow at high speed into the firstcooling vessel. The spraying means may be a nozzle or valve. The outflowvelocity of the melamine melt from the spraying means is, as a rule,greater than 20 m/s, and is preferably greater than 50 m/s.

The cooling vessel contains an ammonia environment and operates at anincreased ammonia pressure. The melamine powder thus formed having atemperature between 100° C. and the solidification point of melamine,preferably between 150° C. and 300° C., most preferably below 270° C.The melamine droplets from the spraying means are cooled by anevaporating cooling medium, for example, liquid ammonia, to producemelamine powder. The melamine melt may contain some portion of liquidammonia with the remaining portion of the liquid ammonia being sprayedinto the first cooling vessel.

The melamine powder formed by spraying the melamine melt into thecooling vessel is held under an increased ammonia pressure, at atemperature above 200° C. for a contact time. The duration of thiscontact time is preferably between 1 minute and 5 hours, more preferablybetween 5 minutes and 2 hours. During this contact time, the temperatureof the melamine product can remain virtually constant or it may becooled to a temperature above 200° C. An increased ammonia pressuremeans an ammonia pressure greater than 1 MPa, preferably greater than1.5 MPa, more preferably greater than 4.5 MPa and even more preferablygreater than 6 MPa. The ammonia pressure is below 40 MPa, preferablybelow 25 MPa and more preferably below 11 MPa.

At the end of the contact time, the melamine powder is cooled to atemperature below 270° C., by mechanically agitating the melamine powderand cooling it directly or indirectly. After the melamine powder hasbeen cooled to a temperature below 270° C., the ammonia pressure isreleased to 0.05-0.2 MPa and, if necessary, the product may be cooledfurther.

The present method may be utilized in both batchwise and continuousprocesses. In the case of batchwise processing, two or more coolingvessels may be used with the melamine melt being sprayed sequentiallyinto the various cooling vessels. Once a first cooling vessel containsthe desired quantity of melamine powder, the spraying means for thefirst cooling vessel can be closed and the spraying means for the secondcooling vessel opened. While the subsequent cooling vessels are beingfilled, the melamine powder in the first vessel can be treated further.In a continuous process, the liquid melamine will generally be sprayedin a first cooling vessel with the accumulating melamine powder beingtransferred into a second cooling vessel where the cooling step takesplace. A hybrid of the batchwise and continuous methods may also beemployed.

The melamine powder must be cooled from a temperature between themelting point of melamine and about 200° C. to a temperature below 100°C. During the spraying step the melamine melt is preferably cooled to atemperature between 10 and 60° C. below the solidification point. Afterthe ammonia pressure has been released, the melamine powder ispreferably cooled by at least 35° C., and more preferably 60° C., bybeing mechanically agitated and cooled directly or indirectly.

Cooling is effected with the aid of an apparatus provided with means forboth mechanically agitating the melamine powder mechanically and forcooling the melamine powder directly or indirectly. Examples of meansfor mechanically agitating the melamine powder include a screw androtating drum, a rotating bowl, rotating discs, rotating segmenteddiscs, rotating pipes and the like.

The melamine powder can be cooled indirectly by contact with the cooledsurface(s) of the fixed and/or moving parts of the cooling apparatus.The fixed and/or moving surface(s) of the apparatus may, in turn, becooled with a cooling fluid such as water or oil. The effective heattransfer coefficient of a suitable cooling apparatus for indirectlycooling melamine powder is preferably between 10 and 300 W/m²K, based onthe cooling area of the apparatus. Preference is also given to the useof a cooling apparatus which comprises means having a cooling area of50-5000 m².

The powder can be cooled directly by a gaseous or evaporating coolingmedium being injected into the cooling vessel, preferably ammonia gas orammonia liquid.

A combination of direct and indirect cooling techniques is preferred forcooling the melamine powder.

Once the melamine powder has been cooled to a temperature below 200° C.,the ammonia pressure may be released. Preferably, ammonia gas iscompletely removed (to an amount below 1000 ppm, preferably below 300ppm, and, most preferably, below 100 ppm) by blowing air through themelamine powder. The ammonia pressure may be released before, or inconjunction with, cooling the melamine powder from a temperature below200° C. to ambient temperature.

The invention will be explained in more detail with reference to thefollowing example.

EXAMPLE

Melamine melt having a temperature of 402° C. is introduced, via aspraying device, into a high-pressure vessel and cooled with liquidammonia which is likewise sprayed in the vessel. The temperature in thevessel is 210° C. The ammonia pressure in the vessel varies between 6.8and 9.2 MPa. The vessel is designed as a rotating drum provided with awall which can be cooled, and provided with a gas inlet. After 2 minutesthe ammonia pressure is released and the melamine powder is cooled toapproximately 50° C. The cooling step to 50° C. took 7 minutes. The endproduct contains 0.4 wt % of melam and less than 0.2 wt % of melem.

COMPARATIVE EXAMPLE

Melamine melt of 400° C., held in a tube under an ammonia pressure of13.6 MPa, is rapidly cooled to ambient temperature by the closed tubebeing brought into contact with a mixture of ice and water. The endproduct contains 1.4 wt % of melam and 0.4 wt % of melem.

What is claimed is:
 1. A method for preparing melamine from urea via ahigh-pressure process by which solid melamine is obtained comprisingsteps: (a) forming a melamine melt, the melamine melt having atemperature between 450° C. and the melting point of melamine; (b)spraying the melamine melt into a vessel, the vessel having an ammoniaenvironment at an elevated ammonia pressure, to form droplets ofmelamine melt; (c) cooling and solidifying the droplets of melamine meltwith an evaporating cooling medium, the evaporating cooling medium beingintroduced into the vessel separately from the melamine melt to formmelamine powder having an intermediate temperature of 270° C. or less;(d) releasing the ammonia pressure and cooling, either directly orindirectly, the melamine powder to a final temperature, the differencebetween the intermediate temperature and the final temperature defininga cooling range; and (e) mechanically agitating the melamine powderduring at least a portion of the cooling range.
 2. A method according toclaim 1, wherein the melamine powder remains under increased ammoniapressure for a contact time of between one minute and five hours, themelamine powder remaining at virtually the same temperature, oroptionally being cooled, during the contact time.
 3. A method accordingto claim 1, wherein the melamine melt is sprayed via spraying meanswithin a vessel into an ammonia environment having a pressure greaterthan 1 MPa.
 4. A method according to claim 1, wherein the ammoniapressure is maintained until the melamine powder reaches an intermediatetemperature below 240° C.
 5. A method according to claim 4, wherein theammonia pressure is maintained until the melamine powder reaches anintermediate temperature below 200° C.
 6. A method according to claim 1,wherein the melamine powder remains under increased ammonia pressure fora contact time of between five minutes and two hours.
 7. A methodaccording to claim 1, wherein the melamine powder remains in contactwith ammonia at a pressure of greater than 1 MPa.
 8. A method accordingto any one of claims 1-7, wherein the melamine powder is cooled by meansof an apparatus provided with means for agitating powder mechanicallyand provided with means for cooling the melamine powder directly orindirectly.
 9. A method according to claim 8, wherein the means formechanically agitating the melamine powder comprises a rotating screw,drum, bowl, discs, disc segments or pipes.
 10. A method according toclaim 8 or 9, wherein the apparatus has an effective heat transfercoefficient of 10-300 W/m²K, based on the cooling area.
 11. A methodaccording to claim 8 or 9, wherein the apparatus has a cooling area of50-5000 m².
 12. A method according to claim 8 or 9, wherein the coolingis carried out at a pressure of 0.05-0.2 MPa.
 13. A method according toclaim 2, wherein the melamine melt is sprayed by a spraying means withina vessel into an ammonia environment at a pressure greater than 1 MPa.14. A method according to claim 2 or 3, wherein the ammonia pressure isreleased if the melamine powder has a temperature below 240° C.
 15. Amethod according to claim 2, 3, 4 or 5, wherein the powder remains incontact with ammonia over a period of five minutes to two hours.
 16. Amethod according to claim 10, wherein the apparatus has a cooling area50-5000 m².
 17. A method for preparing melamine comprising the steps of:reacting urea in a high-pressure melamine reactor to form a melaminemelt, the melamine melt having a temperature between the melting pointof melamine and 450° C.; transferring the melamine melt from themelamine reactor to a first spraying means; feeding the melamine meltthrough the first spraying means into a cooling vessel at a feed ratesufficient to form droplets of melamine melt, the cooling vesselcontaining an ammonia environment at an elevated pressure and atemperature of less than 270° C.; feeding an evaporating cooling mediumthrough a second spraying means into the cooling vessel, the secondspraying means being positioned and configured in such a way as to causethe evaporating cooling medium to intermix with and contact the dropletsof melamine melt; solidifying the droplets of melamine melt to formmelamine powder, the melamine powder having an initial temperature atleast 10° C. less than the solidification temperature at the prevailingpressure; cooling the melamine powder, utilizing direct or indirectcooling, to an intermediate temperature not greater than 270° C., theinitial temperature and the intermediate temperature defining a firstcooling range; maintaining the ammonia pressure within the coolingvessel until the melamine powder has reached the intermediatetemperature, the time during which the ammonia pressure is maintaineddefining a contact period; reducing the ammonia pressure within thecooling vessel after the melamine powder reaches a temperature equal toor below the intermediate temperature; further cooling the melaminepowder from the intermediate temperature to a final temperature of lessthan 50° C., the intermediate temperature and the final temperaturedefining a second cooling range; mechanically agitating the melaminepowder over at least a portion of the second cooling range; and removingthe melamine powder from the cooling vessel.